Yes, it maybe possible, however it probably is not practical. Even with Occlusion Culling, Mipmapping (Varies the resolution of textures based on distance), and LOD (level of detail, varies the complexity of objects based on distance) you are still limited about how much you can load into memory at once, and limited by reasonable estimates on what your customers have available in hardware. It gets even more complex with MMORPG's where the system needs to track the actions of hundreds of players, monsters, and treasure pools. You should check with the Unity forms and see if the engine can even support MMROPGs.
Skyrim, even if it is single player, gets close to that over world concept. They still load dungeons, and some special locations in their own environments, but the entirety of the surface world is free from loading screens. The in game map is really just a orbital camera that has it's Z value locked. What you see in the map is reflective to what actually happening in the game world.
Best Regards,
Avotas